Modulated oscillator



.nected to output coil B. frequency circuit to ground via plates 1, 3 ofthe Patented oet. 13, 1942 Monm'rsp 'oscnm'ron STATES PATENT OFFICE I,

William H. Boghoslan, Forest Bills, and Foster A. Binshaw, St. `Albans,N. Y., and Henry G. Och, West Englewood. N. J., assignors to BellTelephone Laboratories, Incorporated, New N. Y., a corporation of NewYork York,

Application May 3, 1341, Serial No. 391,676

i 3'Clalms. (Cl. 1TB-171.5)

'I'he present invention relates to modulated wave transmission andespecially to a circuit for generating a carrier frequency wave andmodulating the wave by signals such as speech.

4A particular object is a self-oscillating modulator of: simpleconstruction with provision for vsuppressing the unmodulated carriercomponent.

Many well-known types of modulatorsdepend upon some form of balance tosuppress. the carrier, such as a diierential or bridge arrangement oftubes or impedances whereby the unmodulated carrier component appearsacross a circuit element that is inv conjugate relation to the outgoingline. I'

One of the objects of the present inventionA is a carrier suppressionmodulator that depends upona direct shunting of the carrier and that-avoids the necessity. of any bridge or differential arrangement.

The nature and objects of the invention will be more fully disclosed .inthe description to follow,v and from the accompanying drawing in which:

Figs. 1 and 2 are schematic circuit sketches of illustrative embodimentsand Fig. 3 shows impedance diagrams to be referred to in the de- Ascription.

. Fig. 1 shows -a tube I with a crystal 2 which serves the dual purposeof determining the frequency of oscillation and shunting the unmodullated carrier from the load circuit.

The tube I, has its control grid connected to the secondary winding ofinput speech coil 3. and thence through resistor l and condenser 5 inparallel, to the cathode. The anode is concrystal 2. Plates l and 9 arein the circuit of a second grid I0. The circuit osclilates by virtue ofthe coupling within the crystal between the ciicuit 1, l and the circuitl. 3. The circuit The plate has n high oscillates at `the seriesresonant frequency of Y the crystal. When speech is applied from thevoice input circuit through coil 3 to the grid, the carrier waves aremodulated byv .thev action of the tube and the two side-bands aretransmitted through output coil to the line or load circuit II while thecarrier isshunted through the circuit comprising crystal plates 1, 8.

The circuit as shown in Fig. l is provided withnegative feedback for thevoice waves in order to reduce distortion and improve stability ofoperation. A portion of the output side-band is bro .ight back overconductor I2, through loss network I3 to the plate I4 the envelope oftube I. Also, some carrier wave is supplied to the diode plate Il fromplate 9 of the crystal via resistors I5 and I6. Demodulated speechi'lows in the diode circuit in series with resistors IB and 4, thecarrier being by- .passed through condenser 5. Resistor 4, being'feedback.

In Fig. 2, a generally similar circuit is shown withthe circuitproportioning more fully set forth. The tube 20 has its control gridconf,-v nected to speech input coil 3 and its anode connected to outputcoil 6 and crystal 2, sirnilafrly.`

to Fig.l. Feedback of the carrier for producing oscillations-,is throughcondenser 2| to the grid 22, which has a'leak resistor 23 to ground; Thecathode is above ground for direct current bythe drop through biasresistor 24 which has the usual shunting condenser 25. The negativefeedback path is from the plate through condenser 26 to the diode plateIl,4 this feedback being effective at the frequencies of the lowerside-bandand carrier by virtue of impedance network Za as may be seenfrom Fig. 3 where Z: `is indicated as having high impedance at thesefrequencies. The demodulated speech ows through the diode and networksZ3 and Z4 in series. Reference to Fig. 3 shows that Z4 has highimpedancel over the voice range so that a feedback of the demodulatedspeech takes place into the grid circuit, the grid being connected tothe upper orl ungrounded terminal of the net- 4 work Z4.

The network Z2 is designed to have a high im-v pedance at the, desiredoutput frequencies, in

ymodulated carrier component and transmission of the lower side-bandinto the outgoing circuit' II. Circuit II contains a band-pass filter 28for passing the lower side-band and suppressing other frequencies.

A suitable configuration 'of the diode within is given for each o1 lthenetworks Z2, Z3 and Z4. Za.for example,

In the case of network Za, the inductive branchy l gives low impedanceto low frequencies. the capacity branch gives low impedance to highfrequencies while the capacity in commotion with the two other branchesprovides a major. and a minor antiresonant effect suitably spaced anddamped as indicated in Fig. 3. The exact dimensioning will. of course,differ with different frequencies, the Aconfigurations and impedancesbeing illustrative and not limiting.

The circuit details may be varied widely in practice without departingfrom the invention. For example, while the coupling from plate 9 to theoscillator grid is shown as quite simple in character, it may beadvantageous in practice to use other elements in this coupling circuit.in order to secure desired phase shift in particular cases, or toprovide for a voltage step-up. These and other modifications will occurto those skilled in the art. The scope of the invention is defined inthe claims which fol1ow.`

What is claimed is: l

l. In -a carrier wave modulating circuit, a space discharge devicehaving at least triode elenections for impressing the modulated waves onsaid transmission circuit 'and .for suppressing transmission of theunmodulated carrier' com ponent, said latter connections comprising acircuit including electrodes of said crystal in shunt across saidtransmission circuit to provide a substantial short circuit at thecarrier frequency.

2. In a modulating circuit. a space discharge tube having at leasttriode'elements, a crystal having a plurality of plates, a'speech inputcircuit, an outgoing circuit for modulated waves. a coupling from plateto arid through said crystal causing said tube to generate oscillationsof carrier frequency determined by said crystal. connections betweensaid speech inputcircuit and saidl tube-for producing modulation in saidtube of said generated frequency oscillations by the ments, a source ofmodulating waves and a transspeech, means to impressthe modulated waveson said outgoing circuit, and a circuit including a pair of plates ofsaid crystal for shunting the unmodulated carrier component from saidoutgoing circuit.

3. In combination.. a space discharge device having cathode. grid andanode elements. a crysta'l, circuit means coupling said crystal in aresonant path between plate and grid elements cf said'device, saidcoupling means causing said device to generate sustained waves of afrequency determined by the crystal, means including a source of signalwaves coupled to elements of lsaid device to produce modulation of saidwaves.

of themodulated waves, said latter means comprlslng a series resonantcircuit including said crystal connected in shunt across said outgoingcircuit.

WILLIAM H. BOGHOSIAN. FOSTER A. HINSHAW. HENRY G. CCH.

